In 1922, Tydings was elected as a Democrat to the 68th session of the US Congress, and was re-elected to the 69th session, representing the second district of Maryland (March 4, 1923 – March 3, 1927) in the House of Representatives. He was not a candidate for renomination in 1926, having become a candidate for the United States Senate.

He was elected to the Senate in 1926, 1932, 1938 and 1944, and served from March 4, 1927, to January 3, 1951. With Alabama Representative John McDuffie, he co-sponsored the Philippine Independence Act, commonly known as the Tydings–McDuffie Act, which established an autonomous 10-year Commonwealth status for the Philippines. It was planned to culminate in the withdrawal of American sovereignty and the recognition of Philippine Independence.

In 1936, Senator Tydings introduced a bill in Congress calling for independence for Puerto Rico, but it was opposed by Luis Muñoz Marín, an influential leader of the Liberal Party.[2] All the Puerto Rican parties supported the bill. Tydings did not gain passage of the bill.[2] (The US senator had co-sponsored the Tydings–McDuffie Act, which provided independence to the Philippines after a 10-year transition under a limited autonomy.)

During his time in the Senate, Tydings was well known for taking principled, controversial, often unusual stands on various issues. He opposed the New Deal due to his fiscal conservatism, and proposed a constitutional amendment requiring the federal budget to be balanced at all times. He was a strong critic of Prohibition prior to its repeal in 1933.[3]

Following the end of World War II, when the US dropped two atomic bombs on Japan, Tydings sponsored a bill calling for the U.S. to lead the world in nuclear disarmament.[3]

In 1950, he headed a committee, generally known as the Tydings Committee, to investigate Joseph McCarthy's early claims of Communist penetration of the federal government and military. The hearings revolved around McCarthy's charge that the fall of the Kuomintang regime in China had been caused by the actions of alleged Soviet spies in the State Department, and his allegation that the SinologistOwen Lattimore was a "top Russian agent." The hearings, held from March to July 1950, were extremely stormy as charge was met with counter-charge; they attracted much media attention, especially after Louis F. Budenz entered the proceedings as a surprise witness supporting McCarthy's charges. The committee published a report denouncing McCarthy and his claims as a hoax.

When Tydings ran for re-election in 1950, McCarthy's staff distributed a composite picture of Tydings with Earl Browder, the former leader of the American Communist Party. Tydings had never met him before Browder testified in July 1950. The composite photo merged a 1938 photo of Tydings listening to the radio and a 1940 photo of Browder delivering a speech; the text under the composite photo stated that when Browder had testified before Tydings's committee, Tydings had said, "Thank you, sir." Although the quote was technically accurate, it was generally held to be misleading, as it implied a degree of amity between Browder and Tydings that did not exist.

Browder had been subpoenaed to appear before the committee and had been most reluctant to answer questions about allegations of Communist infiltration of the US government. As a result, Tydings and Browder had clashed a number of times, and Tydings's courtesy had come after a lengthy exchange in which Browder had initially refused to answer a question about whether two diplomats had been members of the American Communist Party. In the 1950 election, Tydings was defeated by John Marshall Butler. He became the Democratic nominee for his old Senate seat in 1956, but withdrew before the election for health reasons.

Millard E. Tydings died at his farm, "Oakington", near Havre de Grace, Maryland. He was buried in Angel Hill Cemetery. His gravestone incorrectly gives his Senate election year (1926) as the start of his Senate service, which began in 1927.